In recent years there has been an increased concern in the United States and elsewhere about air pollution from industrial emissions of noxious oxides of nitrogen, sulfur and carbon. Government agencies, in response to such concerns, have in some cases already placed limits on allowable emissions of one or more of the pollutants, and the trend is clearly in the direction of increasingly stringent restrictions. Petroleum fuel refineries are particularly affected by present and anticipated restrictions on emissions, particularly emissions of nitrogen oxides and carbon monoxide.
Although several nitrogen oxides are known which are relatively stable at ambient conditions, it is generally recognized that two of these, viz. nitric oxide (NO) and nitrogen dioxide (NO.sub.2), are the principal contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere. These effects will not be discussed further here since they are well recognized.
Nitric oxide and nitrogen dioxide, under appropriate conditions, are interconvertible according to the equation EQU 2NO+O.sub.2 =2NO.sub.2.
For purposes of the present invention, NO.sub.x will be used herein to represent nitric oxide, nitrogen dioxide (the principal noxious oxides of nitrogen), and/or mixtures thereof.
U.S. Pat. No. 3,900,554 describes a homogeneous gas phase thermal reaction to remove NO.sub.x from combustion effluent by adding 0.4 to 10 moles (preferably 0.5 to 1.5 moles) of ammonia followed by heating to 1600.degree. C. to 2000.degree. C. The NO.sub.X content is lowered as a result of its being reduced to nitrogen by reaction with ammonia. The so-called "selective catalytic reduction" (SCR) type process which operates at a much lower temperature, 200.degree. to 600.degree. C., is exemplified by U.S. Pat. No. 4,220,632, which describes a process for reducing NO.sub.x from a fossil fuel fired power generation plant, or from other industrial plant off-gas streams, to elemental nitrogen and/or innocuous nitrogen oxides by employing ammonia as reductant and, as catalyst, the hydrogen or sodium form of a zeolite having pore openings of about 3 to 10 Angstroms. U.S. Pat. No. 4,220,632 is incorporated herein by reference for its description of selective catalytic reduction (SCR) processes for reducing NO.sub.x emissions.
Monolithic substrates such as wash coated ceramics, described, for example, in U.S. Pat. No. 4,771,029, and extruded catalytic monoliths, described, for example, by Lachman et al., in "Extruded Monolithic Catalyst Supports," Symposium on Catalyst Supports: Chemistry, Forming and Characteristics, American Chemical Society, New York City Meeting, 535-543 (1991), have been described as useful in emissions control.
U.S. Pat. No. 4,800,187 describes a method for crystallizing a zeolite on the surface of a ceramic monolith containing silica with a crystallization mixture containing a ratio of H.sub.2 O/SiO.sub.2 of 16-20 to 1 and a ratio of SiO.sub.2 /Al.sub.2 O.sub.3 of 1 to 0.0-0.4 for the synthesis of ZSM-5. Different ratios are described for large pore zeolites X and Y.
Now it has been found that the structure of a catalyst comprising a zeolite and substrate support improves the efficacy of catalytic NO.sub.x reduction.
The catalyst structure for use herein and the method for its manufacture are novel and provide an improved NO.sub.x abatement process.